This invention relates to non-destructive testing, and more particularly to the use of ultrasonics to perform extremely high resolution testing of the properties of a material.
Non-destructive testing of material properties is well-known in the art. One method of non-destructive testing is the use of ultrasonic waves. This method is used, for example, to test the material's thickness, and detect flaws in the material. The method of ultrasonic testing involves generation of an ultrasonic waveform, propagating the waveform into the material, detecting a return, reflected waveform or echo, and processing the echo waveform to determine parametric values.
Previous methods of ultrasonic testing have involved the measurement of a particular characteristic of the ultrasonic waveform. These include detecting a zero-crossing of the waveform, or detecting when the amplitude of the waveform exceeds some predetermined threshold, or detecting when the waveform amplitude reaches a peak or maximum value. Various methods have been used to perform measurements using ultrasonic waves. One such method involves use of a stable, free running oscillator having a known and accurate frequency. Pulses from the oscillator are counted by a counter which is started and stopped in response to a sensed characteristic of the ultrasonic waveform. The resultant count, for example, represents the ultrasonic signal's time of flight (TOF); i.e., the transit time for the waveform to be propagated into the test object and the return echo detected. This "single shot" testing method is satisfactory, but it does have some drawbacks. It is limited in resolution to measurements of approximately 0.001 inches, or a time of flight of approximately 8 nanoseconds (8*10.sup.-9 sec.). Because it involves a single sample, there are limits as to the accuracy of the measurement. Accuracy can be improved by a factor of ten by taking multiple samples and developing an arithmetic average value for the samples taken. But, this may require more time than is available for the measurement.
One approach utilizes a series of analog measurements. Here, an integrator accumulates a voltage over a given time window. The integrated voltage value is then measured using a digital voltmeter. The result is an accurate measurement, but one which is effected by the amount of drift in the analog circuits. Other alternate approaches employ other combinations of digital and analog circuits and techniques. In general, use of analog techniques provides some interpolation of direct digital techniques to improve the accuracy of the measurements.
Further with respect to current processing techniques, it is common to measure time of flight to a fixed threshold level of the waveform; for example, 50% of the full output of a screen on which the waveform is monitored. Problems arise because normal variations in the amplitude of the waveform will move this 50% point due to changes in the slope of the waveform. This, in turn, effects the time of flight calculation. Where time of flight is based upon reference to a zero crossing of the waveform, slight variations in the waveform caused by its lower frequency components will also effect the accuracy of the time of flight calculations.
Despite the measurement accuracy which is achievable with existing techniques, there is a practical limit beyond which improvements are not easily achieved. This presents a major concern where very small measurements are required and where a high degree of accuracy is needed. In nuclear reactors for example, a zirconium alloy tubing is used. Not only is the material expensive, but the tubing is a very small tubing, having a wall thickness which must be measured to an accuracy on the order of 10 microinches (10*10.sup.-6 in.). Because of the amount of zirconium tubing used in a reactor facility, it represents a substantial cost. Also, because of the nature of the facility, it must be precisely measured to insure its suitability for installation. If the diameter of the tubing cannot be precisely measured, if its inner and outer diameters and eccentricity cannot be accurately determined, otherwise acceptable tubing may be unnecessarily rejected. A precise measurement technique and method which will enable the physical characteristics of the tubing to be precisely determined, during volume production of the tubing, might lower the reject rate of the tubing. This, in turn, would not only insure that suitable tubing were accepted for use in the facility, but also that the overall manufacturing costs of the tubing are lowered.
Finally, with respect to the noted problems caused by threshold and zero crossing variations, there is need for a technique by which any variations are readily accounted for without impacting the speed with which accurate data is obtained.